The use of simulcast transmission to increase the effective radio frequency (RF) coverage area of land mobile radio communications systems is well known in the art. In simulcast transmission, two or more transmitters, simultaneously broadcasting identical information on the same frequency, are geographically located such that contiguous coverage is available over a larger area than can be covered by one transmitter acting alone. In order to insure that identical signals broadcast from separate transmitters do not interfere with each other, most simulcast transmission systems require that the base band signals be transmitted at a precisely controlled time from all of the remote site transmitters. If the signal is transmitted from one of the transmitters at the wrong time, distortion might occur in the area where signals from both transmitters are received. Such distortion might occur if identical signals from two or more transmitters arrive at a receiver, at slightly different times, thereby slightly out-of-phase with respect to each other. Furthermore, in order for a received simulcast signal to be intelligible, the remote site transmitters must all be modulated with substantially the same signal at substantially the same point in time. That is, the remote sites must all be synchronized with respect to each other.
In general, a simulcast system is comprised of a prime site, multiple remote transmitter sites, and subscriber units. A modulated signal is sent, typically via a microwave distribution network, from the prime site to the remote transmitter sites. At the remote site, if the information the prime site is distributing is digital information, the digital data is recovered using any of a variety of known clock recovery schemes. After recovering the data, transmission of the data from the remote sites must then be synchronized for simultaneous transmission from all the remote sites. After synchronization, the remote sites transmit a modulated signal to the subscriber unit in the simulcast coverage areas.
Todays simulcast systems that broadcast digital information on an FM carrier, F.sub.c, typically utilize a binary, or two level modulation scheme wherein for example, a binary digit of one, having a predetermined bit time, might be represented by an RF signal having the same bit time at a frequency equal to F.sub.c +4000 Hz. and a binary zero might be represented by an RF signal having the same bit time at a frequency equal to F.sub.c -4000 Hz. Such a two-level modulation scheme involves modulating a carrier between two frequencies, such as the two aforementioned frequencies, F.sub.c +4000 and F.sub.c -4000, for discrete periods of time, wherein each bit time of the data stream is represented by a corresponding amount of time and a corresponding frequency.
When using a two level modulation scheme, the synchronization is typically achieved by using finite time delays between the prime site and the remote sites, each of which time delay is programmed into the remote site transmitters. That is, the remote sites, which receive the data and then recover it, will each wait a predetermined amount of time before transmitting the data to the subscriber. This fixed time delay, which varies amongst the remote sites, attempts to insure simultaneous transmission of the data, but can only achieve simultaneous transmission within a predetermined tolerance level. So long as the data is properly recovered at the remote site and synchronized with the local clock at the transmitter site, any synchronization of the remote sites with each other, which is not accounted for by accurate delay parameters, will be innocuous with respect to the intelligibility of the received signal.
The aforementioned synchronization scheme is adequate for two level modulation systems because the transmitter's modulation is not particularly sensitive to individual bit pairs, or di-bits, in the signal to be broadcast. However, two level modulation systems are limited in their simulcast performance, since the delay tolerance is directly related to the bit time. In contrast, a four level modulation scheme may be used to extend the delay tolerance of the system, because the delay tolerance capability is directly related to twice the bit time (i.e., di-bit, or symbol, time). In a four-level modulation system, digital bits in a stream of such information are grouped into bit pairs. Since there are at most four combinations of ones and zeroes in a pair of ones and zeroes (i.e., 00, 01, 10, 11), paired ones and zeroes in a stream of such data can be mapped into four modulation levels. Instead of having two frequencies to represent two digits, (i.e. 0 and 1), four frequencies are used to represent the four possible combinations of one's and zeroes in pairs of such digits. For example, F.sub.c +4000 might represent binary 10; F.sub.c +2000 might represent binary 11; F.sub.c -2000 might represent binary 01; and F.sub.c -4000 might represent binary 00. When using a four-level modulation, the time that the modulated carrier signal is at a particular frequency is ordinarily twice the bit time of the bits from the underlying data stream because two bits from the data stream are being mapped into one of four modulation levels. In a four level scheme however, the synchronization of the ones and zeroes of the data becomes a critical parameter for a successful simulcast transmission. This is due to the fact that, in addition, to clock/data recovery, each of the remote site transmitters must modulate the same two-bit pairs at precisely the same time in order to produce an intelligible, four level transmission.
Four level data in a four-level modulation system is typically grey coded (i.e., only one bit in adjacent levels is permitted to change its binary state), which coding rules are shown in Table 1.
TABLE 1 ______________________________________ Level Binary Code Grey Code ______________________________________ 1 00 00 2 01 01 3 10 11 4 11 10 ______________________________________
FIG. 1A shows a binary data stream 100 which may be grey coded in accordance with the rules set forth in Table 1. Binary data stream 150 is simply the grey coded equivalent of binary data stream 100. The binary data of data stream 100 is first partitioned into two-bit pairs, 102-112, which are then grey coded into pairs 102'-112'. FIG. 1B shows a mapping diagram 180, which illustrates how data stream 100 could appear as a grey coded, four level, amplitude modulated signal. (Pictorially showing FM modulation levels or deviation levels is not readily accomplished.) In a four-level modulation system, each transmitter must broadcast precisely the same signal shown in this mapping diagram for the simulcast transmission from each of the remote sites to be intelligible.
FIG. 1C shows a mapping diagram 190 illustrating the consequences of improper bit-pair modulation. In FIG. 1C, synchronization is off by one bit position. Even if the data in the data stream is fully recovered and synchronized to the respective internal clocks at each remote site, each remote site must begin bit pair modulation on the same bit, and as shown in FIG. 1C, on bit pair 102. As an example, suppose that a remote site A modulates the binary data stream 150, generating the mapping diagram shown in FIG. 1B. Alternatively, a second remote site B begins the mapping process on an odd numbered bit, resulting in the mapping diagram 190 shown in FIG. 1C. When these two modulated signals are launched from their respective remote site transmitters, the received signal in the simulcast coverage area might be unintelligible. In contrast to the two level case where such a modulation error simply causes a signal phase anomaly (i.e., transmission begins one or more clock pulses later), in a four level modulation scheme the error has a much greater impact. In particular, the modulated data stream transmitted by remote site A (data pairs 102'-112') is drastically different from that transmitted from remote site B (bit pairs 103-111). A subscriber in the simulcast coverage area might simultaneously receive the modulated signals 180 and 190, but would only recover unintelligible audio.
Existing microwave distribution methods typically transport either a waveform in the case of analog signals, or a two level modulated signal in the case of digital microwave. The actual transport signal used on analog microwave systems is a two level signal. Although it is possible for an analog system to transport a four level waveform, there is presently no equipment available to perform a two-level to four-level encoding at the prime site. In using a four-level modulation it would be highly desirable to have existing, single transport signals useable for both two-level and four-level signals. Since existing digital microwave systems generally transport only a two level signal, an improved synchronization method is required so that two level signals can be properly modulated onto a four level transport signal.
Accordingly, there exists a need for a synchronization scheme which may be employed by a remote site transmitter and a simulcast transmission system which transmits binary data in a four level modulated form. Such a scheme should also be compatible with two level simulcast transmission systems, while not being limited to exclude those systems which may partially employ four level modulation.